You could always grab pictures off the web, print them out on a good color printer, and take the printouts to your local copy shop to have them enlarged to a minimum of 11x17 on the color copier. You can probably go larger if you you are willing to pay the extra cost (in fact, they have oversized pro inkjet ones that can do HUGE prints, but it's very expensive).
You could also take your own pictures on a digital camera - make sure you use the highest resolution setting. Then - if your printer supports this - you can print out wall sized posters by tiling it, printing several pages that when combined make the total image.